Biological fouling is a significant problem in a wide variety of fluid handling systems. Traditional methods may not be effective at detecting a biofilm before irreversible damage is done.
In particular, both the vulnerability of water systems to bioterrorism and the increasing pressure on a limited fresh water resource require techniques for preventing infection and toxicity of water supplies. Because typhoid is a more acutely obvious problem than cancer or birth defects, society, at least in the U.S., has compromised potential long-term safety by emphasizing short-term protection from infection. Continuous pathogen suppression with biocides to control contamination in the waste discharges and drinking water distribution systems leads to exposure to low levels of established toxins/carcinogens from disinfection byproducts. Substitution of high-energy exposure (e.g. ultraviolet light or ozone) treatments at water treatment facilities for continuous chlorination can provide safe water that is free of disinfection byproducts, but can chemically modify drinking water refractory organic constituents, thereby promoting microbial regrowth in distribution systems if the water treatment or distribution system integrity allows refractory organics into the drinking water. This protection requires a much more effective water treatment.
It has been demonstrated that regrowth leads to biofilm formation, and that biofilms can both nurture pathogenic microbes and offer protection from biocides. The biofilms also concentrate drugs, hormones, and their mimics, as well as other pharmacologically active pollutants. These microbial biofilms in the distribution system can be major threats to freshwater reuse if water use for reuse is insufficiently treated to remove most organic carbon and trace nutrients.
Microbes in nature and in drinking water distribution systems are mostly concentrated in multispecies community biofilms rather than floating freely in fluids. A logical and cost-effective method for sampling water microbes is to incubate strategically placed coupons that stimulate colonization in hours to days rather than weeks. The coupons are subsequently recovered for analysis of the microbial biofilms from the waters, after which any required treatment is applied to the water. Reproducible generation of biofilms that can be infected and colonized by pathogens and concentrate some hydrophilic drug/hormone components has been demonstrated.
Several approaches to detection of biofilms have been demonstrated in the laboratory, which approaches are summarized in Table 1. Infrared absorption based on Attenuated Total Reflection (ATR) spectroscopy has been used to monitor the conditioning films that are an early harbinger of biofilm formation. ATR has also been studied for detection of suspended and biofilm bacteria. However, because of the limited depth of penetration of the evanescent wave into the surrounding medium, less than 1 micron of the film can be interrogated. Also, the resultant spectra are very complicated and convoluted because of the overlay of vibrational spectroscopic contributions from all the molecules in the interrogation region (Nivens et al., 1995). Many researchers have used fiber optic probes to transmit light to a solution or biofilm (Mittelman et al, 1993; Anders et al., 1993). Earlier research demonstrated that bacteria adhering to an unclad optical fiber can be detected in low numbers based on refractive index discontinuity (Tabacco, 1994). Fiber optics have also been used to measure backscattered light intensity in biofilms (Beynal et al., 2000). In this case the tapered fiber probes actually penetrate the biofilm. Backscattered near infrared light is correlated with local diffusivity as measured by a microelectrode, which in turn is correlated with the density of the biofilm. However, this technique does not specifically distinguish biological from nonbiological films that arise from mineral deposits.
TABLE 1Technologies Applied to Biofilm DetectionEvanescent wave—ATRLimited depth of penetration;convoluted spectraFiber optic—refractive indexMay not discriminate biological/non-bioMass—QCMNo discrimination, many interferentsElectrochemical—impedance,Indirect measurement/non-biologicalconductanceFluorescence—direct UVExcellent sensititvity; specific forexcitationbio-fouling
Similarly, piezoelectric sensors such as quartz crystal microbalances monitor frequency shifts as mass accumulates on the sensor surface. They have been used to detect biofilms, but suffer from pressure and temperature interferences, and are responsive to any type of material (e.g., biological or abiological) landing on the surface (Nivens et al, 1993).
NASA has demonstrated a microscope based imaging system for monitoring the attachment and detachment of organisms in a biofilm upon application of biocide (Pierson et al., 2000).
Traditional methods have been ineffective at detecting a biofilm before fouling leads to disruptions in mechanical operations, losses in heat transfer efficiency, or materials deterioration. Alto, there are no real-time in situ models for monitoring biofilms that pose serious health risks to individuals in enclosed spaces, such as during long duration space missions. Continuous monitoring of surfaces for biofilm formation would represent a major advance in prevention, mitigation, and treatment efforts, and should greatly reduce personnel time and costs associated with adverse effects arising from biofilms.